The primary objectives of this morphometric study is to provide quantitative data about the later stages of fetal lung development in the rabbit. The proposed research will examine, from a structural viewpoint, the generality of a recent conclusion that antenatal glucocorticoid administration accelerates lung maturity in monkey lungs by inducing structural changes in the connective tissue of the lung. Earlier animal studies have indicated that an adequate amount of surface active material on the alveolar surface of the lung is of primary importance for the normal function of the lungs in newborns and the prevention of respiratory distress syndrome in prematures. This study will be carried out at the light microscopic and the electron microscopic levels. The morphometric evaluations will include: the volume of the parenchyma, surface area of the parenchyma, number of saccules and type II cells in the lung, and the volume of lamellar bodies in the type II cells. Five developmental sequences will be compared: normal development of the fetal lung, and altered development of the fetal lung after administration of glucocorticoids, suppression of endogenous glucocorticoids, administration of thyroxine, and inhibition of thyroid function.